The present invention relates to an apparatus for measuring the cross-sectional distribution of the refractive index of an optical waveguide used for optical communication.
To measure the cross-sectional distribution of the refractive index of an optical waveguide, there is a refracted near field method (RNF method). This RNF method features high measuring accuracy and high resolution and is considered to be currently the most superior method of measuring the cross-sectional distribution of the refractive index on an optical waveguide.
In the RNF method, as shown in FIG. 9, an optical waveguide substrate 1, which comprises a substrate portion 2 and an optical waveguide portion 3 formed on one surface of the substrate portion 2, is immersed in liquid 9 having a refractive index nL near the refractive index n(r) of the optical waveguide portion 3. In this state, a laser beam converged by an objective lens 8 is caused to be incident at an incident angle .theta. on an end face of the optical waveguide portion 3, and the light leaking from the optical waveguide portion 3 is detected to measure the refractive index of the optical waveguide portion 3.
When n(r) represents the refractive index of the optical waveguide portion 3 at the point of incidence of the laser beam, and n.phi. is the refractive index of air or liquid on the incidence side of the optical waveguide portion 3, the exit angle .beta. corresponding to the incident angle .theta..phi. is defined by the Snell's law and simply given as the following equation 1. EQU n.sup.2 (r)=n.phi..sup.2. sin.sup.2 .theta..phi.+nL.sup.2.cos.sup.2 .beta.(1)
Thus, by scanning the point of incidence of the laser beam in the thickness direction of the optical waveguide portion 3 or in the perpendicular direction to the thickness, the exit angle .beta. is changed according to the refractive index n(r) at each point. More specifically, the exit angle .beta. is reduced with increasing refractive index and increased with reducing refractive index.
Thus, it is possible to know the refractive index n(r) of the optical waveguide portion 3 by judging the state of leaking light.
The apparatus for measuring the cross-sectional distribution of the refractive index of an optical waveguide by the RNF method, is based on the above principle.
Referring to FIG. 9 again, a detector 5 for receiving light leaking from the optical waveguide portion 3 is provided sidewise of the optical waveguide substrate 1. Further, a semi-circular shielding plate 6 is provided such that it blocks a central portion of the leaking light flux 4. The detector 5 thus receives leaking light 4 having a half-doughnut-like sectional profile lacking a central portion. The light receiving quantity P is given as the following equation 2, where .theta..phi.max represents the exit angle of the outermost light receiving point of the leaking light, and .theta..phi.min represents exit angle of the innermost light receiving point shielded by the shielding plate 6. ##EQU1##
In the above equation, I(.theta..phi.) represents the angle dependency intensity distribution of the incident light. The light receiving surface of the detector 5 is made sufficiently large lest the leaking light 4 should get out of the light receiving surface. Thus, .theta..phi.max in the equation 2 is determined by the numerical aperture (NA) of the objective lens and is given as the following equation 3. EQU n.theta..phi.. sin.theta..phi.max=NA (3)
The exit angle .beta.max is changed, i.e., the outermost light receiving point of the leaking light is shifted, according to the refracting power of the optical waveguide portion 3, but the exit angle .beta.min is determined absolutely by the positions of the edge of the shielding plate 6, and is not influenced by the refractive index of optical waveguide portion 3.
The incident angle .theta..phi.min corresponding to the exit angle .beta.min can be obtained by the equation 4, which is a rearrangement of the above equation 1. EQU n.phi..sup.2. sin.sup.2 .theta..phi.min=n.sup.2 (r)-nL.sup.2. cos.sup.2 .beta.min (4)
The incident angle .theta..phi.min is an important factor in determining the refractive index of the optical waveguide portion 3. That is, the quantity of light that can be obtained by the equation 2 is changed according to the refractive index.
When it supposed that light receiving quantity at a driven point in the thickness direction of the optical waveguide portion 3 or in the perpendicular direction to the thickness is P(n(r)), this light receiving quantity P(n(r)) is given as the following equation 5 . ##EQU2##
Next, if the angle dependency I(.theta..phi.) of the incident light intensity has a Lambert distribution [I(.theta..phi.)=I.phi. cos.theta..phi.], by scanning the laser spot position in the thickness direction of the optical waveguide portion or the perpendicular direction to the thickness and by measuring the change of light quantity .DELTA.P, the following equation 6 can be obtained from the above equation 5, and .DELTA.n(r) can be obtained from the equation 6. EQU .DELTA.P=a..DELTA.n(r) (6)
In this equation, the proportionality constant a is determined by the known refractive index n.sub.L.
As the light source, a laser is usually used. In this case, the incident light intensity distribution I(.theta..phi.) is a Gauss distribution rather than a Lambert distribution, and change in the light quantity and change in the refractive index are not so simple as the equation 6. However, it is possible to obtain .DELTA.n(r) by correction by calculation.
In the above conventional type apparatus for measuring the cross-sectional refractive index distribution of an optical waveguide, the substrate portion of the optical waveguide is immersed in a liquid having a refractive index near, preferably higher than, that of the optical waveguide substrate portion in order to prevent total reflection of light in the optical waveguide portion and permit effective leaking of light incident on the optical waveguide portion to the outside thereof. That is, in the conventional type apparatus immersion liquid is indispensable.
In the case of an optical waveguide portion made of glass, the refractive index is about 1.5, and the immersion liquid can be comparatively readily selected. However, in many cases where an optical waveguide portion is formed by thermally diffusing Ti in a monocrystalline substrate of LiNbO.sub.3, LiTaO.sub.3, etc., the substrate portion has a refractive index of 2.0 or above.
With such optical waveguide substrate an immersion liquid having substantially the same refractive index is necessary. However, liquid with a refractive index of 2.0 or above is harmful to the human body. Further, depending on the material of the substrate, there may be no adequate immersion liquid even if the refractive index is low. Therefore, depending on the material of the optical waveguide portion the measurement is difficult or hazardous, or it is impossible.
The present invention has been intended in view of the above circumstances, and it seeks to permit the measurement of the refractive index distribution of optical waveguides without any immersion liquid.
The applicant has earlier proposed an another system for solving the problems pertaining to the invention in Japanese Patent Application 15481/1991.